Low surface roughness cast strip and method and apparatus for making the same

ABSTRACT

A thin cast strip is formed having at least one microstructure selected from the group consisting of polygonal ferrite, acicular ferrite, Widmanstatten, bainite and martinsite, a surface roughness of less than 1.5 microns Ra and a scale thickness of less than about 10 microns by applying a mixture of water and oil on the work rolls of the hot rolling mill, passing the thin cast strip at a temperature of less than 1100° C. through the hot rolling mill while the mixture of oil and water is applied to the work rolls, and shrouding the thin cast strip from the casting rolls through the hot rolling mill in an atmosphere of less than 5% oxygen to form the thin cast strip.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to cast strip made by a twin roll caster, and themethod and apparatus for making such cast strip.

In a twin roll caster, molten metal is introduced between a pair ofcounter-rotated horizontal casting rolls which are cooled so that metalshells solidify on the moving roll surfaces, and are brought together atthe nip between them to produce a solidified strip product delivereddownwardly from the nip between the casting rolls.

The term “nip” is used herein to refer to the general region at whichthe casting rolls are closest together. The molten metal may be pouredfrom a ladle through a metal delivery system comprised of a tundish anda core nozzle located above the nip to form a casting pool of moltenmetal supported on the casting surfaces of the rolls above the nip andextending along the length of the nip. This casting pool is usuallyconfined between refractory side plates or dams held in slidingengagement with the end surfaces of the rolls so as to dam the two endsof the casting pool against outflow.

When casting steel strip in a twin roll caster, the strip leaves the nipat very high temperatures on the order of 1400° C. or higher. If exposedto normal atmosphere, it would suffer very rapid scaling due tooxidation at such high temperatures. Therefore, a sealed enclosure isprovided beneath the casting rolls to receive the hot strip and throughwhich the strip passes on the way from the strip caster, the enclosurecontaining an atmosphere which inhibits oxidation of the strip. Theoxidation inhibiting atmosphere may be created by injecting anon-oxidizing gas, for example, an inert gas such as argon or nitrogen,or combustion exhaust gases which may be reducing gases. Alternatively,the enclosure may be sealed against ingress of oxygen containingatmosphere during operation of the strip caster. The oxygen content ofthe atmosphere within the enclosure is then reduced during an initialphase of casting by allowing oxidation of the strip to extract oxygenfrom the sealed enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and5,960,855.

Cast strip produced by twin roll caster is generally hot rolled in a hotrolling mill after the strip emerges from the caster to shape the thinstrip. It is generally understood that to cast strip with acceptableshape, the hot rolling mill is used in connection with a twin rollcaster is to provide the desired cross-sectional profile to the caststrip. Still, a surface roughness of 6 to 8 microns Ra with surfacemicro-cracking was common for cast strip emerging from the hot rollingmill while casting at a standard casting speed of 80 m/min and 16%reduction of the strip by the hot rolling mill. FIG. 1 is a micrographshowing typical surface roughness of such cast strip emerging from a hotrolling mill in-line with a twin roll caster. With the direction ofrolling from left to right, the micrograph shows pronounced lapping onthe strip surface (20 to 30 μm deep). The reason or reasons for thissurface roughness may be shearing at the strip surface caused by weldingof the strip to the work roll surface, imprinting of the texture of thework roll surface onto the surface of the strip, and/or other factors.Moreover, micro-cracking on the surface of the cast strip was a problem.It was possible to reduce the casting speed and the heat rate of thestrip to reduce microcracking, but it was uneconomical to reproducethese conditions during production.

The microstructure of hot strip mill products is essentially 100%equiaxed ferrite. However, in making a cast strip with a twin rollcaster, previous experience was that microstructure was coarse grains ofpolygonal ferrite, acicular ferrite, and Widmanstatten. It was typicalthat the microstructure was 30-60% polygonal ferrite, 70-40%Widmanstatten and acicular ferrite. With this microstructure, thetypical surface roughness was 4-7 microns Ra.

A thin cast strip is provided having at least one microstructureselected from the group consisting of polygonal ferrite, acicularferrite, Widmanstatten, bainite and martinsite, a surface roughness ofless than 1.5 microns Ra and a scale thickness of less than about 10microns made by the steps comprising:

a. assembling a twin roll caster having laterally positioned casterrolls forming a nip between them and a hot rolling mill having workrolls and back-up rolls adjacent the twin roll caster,

b. forming a thin cast strip from the nip between the casting rolls ofthe twin roll caster,

c. applying a mixture of water and oil on the work rolls of the hotrolling mill,

d. passing the thin cast strip at a temperature of less than 1100° C.through the hot rolling mill while the mixture of oil and water isapplied to the work rolls, and

e. shrouding the thin cast strip from the casting rolls through the hotrolling mill in an atmosphere of less than 5% oxygen forming a caststrip having at least one microstructure selected from the groupconsisting of polygonal ferrite, acicular ferrite, Widmanstatten,bainite and martinsite, a surface roughness of less than 1.5 microns Ra,and a scale thickness of less than about 10 microns.

A thin cast strip may have a surface roughness less than 1.0 microns Raor less than 0.7 or 0.5 microns Ra. The thin cast strip may have a scalethickness less than 7 or 4 microns. The cast strip may be passed throughthe hot rolling mill at a temperature less than 1050° C. while a mixtureof oil and water is applied to the work rolls. The mixture of oil andwater may be applied by spraying the work rolls, or may be applied tothe back-up rolls. The mixture of oil and water may be less than 5% oilto form a thin cast strip with a low surface roughness of less than 1.5microns Ra.

BRIEF DESCRIPTION OF THE DRAWINGS

The operation of an illustrative twin roll casting plant in accordancewith the present invention is described with reference to theaccompanying drawings, in which:

FIG. 1 is a micrograph showing typical surface roughness of a cast stripafter hot rolling;

FIG. 2 is a schematic illustrating a thin strip casting plant having ahot rolling mill for controlling the shape of cast strip;

FIG. 3 is an enlarged cut-away side view of the caster of the thin stripcasting plant of FIG. 2;

FIG. 4 is a schematic diagram showing a system for the application of anoil and water mixture to the rolls of a hot rolling mill; and

FIG. 5 is a diagram showing the Average Surface Roughness for Thin CastSteel Strip, Sequence 2613, made using o the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The illustrated casting and rolling installation comprises a twin-rollcaster denoted generally by 11 which produces thin cast steel strip 12which passes into a transient path across a guide table 13 to a pinchroll stand 14. After exiting the pinch roll stand 14, thin cast strip 12passes into and through hot rolling mill 15 comprised of back-up rolls16 and upper and lower work rolls 16A and 16B, where the thickness ofthe strip reduced. The strip 12, upon exiting the rolling mill 15,passes onto a run out table 17 where it may be forced cooled by waterjets 18, and then through pinch roll stand 20 comprising a pair of pinchrolls 20A and to a coiler 19.

Twin-roll caster 11 comprises a main machine frame which supports a pairof laterally positioned casting rolls 22 having casting surfaces 22A andforming a nip between them. Molten metal is supplied during a castingcampaign from a ladle (not shown) to a tundish 23, through a refractoryshroud to a removable tundish 25 (also called distributor vessel ortransition piece), and then through a metal delivery nozzle 28 (alsocalled a core nozzle) between the casting rolls 22 above the nip.

Molten steel is introduced into removable tundish 25 from tundish 23 viaan outlet of the refractory shroud. The tundish 23 is fitted with astopper rod and a slide gate valve (not shown) to selectively open andclose the outlet of the shroud and effectively control the flow ofmolten metal from the tundish 23 to the caster. The molten metal flowsfrom removable tundish 25 through an outlet and optionally to andthrough the delivery nozzle 28.

Molten metal thus delivered to the casting rolls 22 forms a casting poolabove nip supported by casting roll surfaces 22A. This casting pool isconfined at the ends of the rolls by a pair of side dams or plates,which are applied to the ends of the rolls by a pair of thrusters (notshown) comprising hydraulic cylinder units connected to the side dams.The upper surface of the casting pool (generally referred to as the“meniscus” level) may rise above the lower end of the delivery nozzle 28so that the lower end of the deliver nozzle is immersed within thecasting pool.

Casting rolls 22 are internally water cooled by coolant supply (notshown) and driven in counter rotational direction by drives (not shown)so that shells solidify on the moving casting roll surfaces and arebrought together at the nip to produce the thin cast strip 12, which isdelivered downwardly from the nip between the casting rolls.

Below the twin roll caster 11, the cast steel strip 12 passes within asealed enclosure 10 to the guide table 13, which guides the strip to apinch roll stand 14 through which it exits sealed enclosure 10. The sealof the enclosure 10 may not be complete, but is appropriate to allowcontrol of the atmosphere within the enclosure and access of oxygen tothe cast strip within the enclosure as hereinafter described. Afterexiting the sealed enclosure 10, the strip may pass through furthersealed enclosures after the pinch roll stand 14, including the hotrolling mill 15.

Enclosure 10 is formed by a number of separate wall sections that fittogether at various seal connections to form a continuous enclosurewall. These sections comprise a first wall section 41 at the twin rollcaster to enclose the casting rolls 22, and a wall enclosure 42extending downwardly beneath first wall section 41 to form an openingthat is in sealing engagement with the upper edges of a scrap boxreceptacle 40. A seal 43 between the scrap box receptacle 40 and theenclosure wall 42 may be formed by a knife and sand seal around theopening in enclosure wall 42, which can be established and broken byvertical movement of the scrap box receptacle 40 relative to enclosurewall 42. Seal 43 is formed by raising the scrap box receptacle 40 tocause the knife flange to penetrate the sand in the channel to establishthe seal.

This seal 43 can be broken by lowering the scrap box receptacle 40 fromits operative position, preparatory to movement away from the caster toa scrap discharge position (not shown). Scrap box receptacle 40 ismounted on a carriage 45 fitted with wheels 46 which run on rails 47,whereby the scrap box receptacle can be moved to the scrap dischargeposition. Carriage 45 is fitted with a set of powered screw jacks 51operable to lift the scrap box receptacle 40 from a lowered position,where it is spaced from the enclosure wall 42, to a raised positionwhere the knife flange penetrates the sand to form seal 43 between thetwo.

Sealed enclosure 10 further may have a third wall section disposed 61about the guide table 13 and connected to the frame of pinch roll stand14, which includes a pair of pinch rolls 50. The third wall sectiondisposed 61 of enclosure 10 is sealed by sliding seals.

Most of the enclosure wall sections 41, 42 and 61 may be lined with firebrick. Also, scrap box receptacle 40 may be lined either with fire brickor with a castable refractory lining. In this way, the completeenclosure 10 is sealed prior to a casting operation, thereby limitingaccess of oxygen to thin cast strip 12, as it passes from the castingrolls 22 through the pinch roll stand 14 and the hot rolling mill 15.Initially the strip can take up all of the oxygen from enclosure 10space by forming heavy scale on an initial section of the strip.However, the sealing enclosure 10 limits ingress of oxygen into theenclosure from the surrounding atmosphere to below the amount of oxygenthat could be taken up by the strip. Thus, after an initial start-upperiod, the oxygen content in the enclosure 10 will remain depleted solimiting the availability of oxygen for oxidation of the strip 12. Inthis way, the formation of scale is controlled to a thickness less than10 microns without the need to continuously feed a reducing ornon-oxidizing gas into the enclosure. Of course, a reducing ornon-oxidizing gas may be fed through the enclosure walls. However, inorder to avoid the heavy scaling during the start-up period, theenclosure 10 can be purged immediately prior to the commencement ofcasting so as to reduce the initial oxygen level within enclosure 10,thereby reducing the time period for the oxygen level to stabilize inthe enclosure as a result of the interaction of the oxygen in oxidizingthe strip passing through it. Thus, illustratively, the enclosure mayconveniently be purged with, for example, nitrogen gas. It has beenfound that reduction of the initial oxygen content to levels of between5% will limit the scaling of the strip at the exit from the enclosure 10to about 10 microns to 17 microns even during the initial start-upphase. In an embodiment of the present invention, the thin cast steelstrip has a scale thickness less than about 10 microns, or the scalethickness may be less than 7 or 4 microns, during continuous casting.

At the start of a casting campaign, a short length of imperfect strip isproduced as the casting conditions stabilize. After continuous castingis established, the casting rolls 22 are moved apart slightly and thenbrought together again to cause this lead end of the strip to break awayin the manner described in Australian Patent 646,981 and U.S. Pat. No.5,287,912, to form a clean head end of the following thin cast strip 12.The imperfect material drops into scrap box receptacle 40 locatedbeneath caster 11, and at this time swinging apron 38, which normallyhangs downwardly from a pivot 39 to one side of the caster as shown inFIG. 3, is swung across the caster outlet to guide the clean end of thincast strip 12 onto the guide table 13 where the strip is fed to thepinch roll stand 14. Apron 38 is then retracted back to its hangingposition as shown in FIG. 3 to allow the strip 12 to hang in a loop 36beneath the caster as shown in FIGS. 2 and 3 before the strip passesonto the guide table 13. The guide table 13 comprises a series of stripsupport rolls 37 to support the strip before it passes to the pinch rollstand 14. The rolls 37 are disposed in an array extending from the pinchroll stand 14 backwardly beneath the caster and curve downwardly tosmoothly receive and guide the strip from the loop 36.

The twin-roll caster may be of a kind which is illustrated and describedin detail in U.S. Pat. No. 5,184,668 and 5,277,243, or U.S. Pat. No.5,488,988. Reference may be made to these patents for constructiondetails, which are no part of the present invention.

Pinch roll stand 14 comprises a pair of pinch rolls 50 reactive totension applied by the hot rolling mill 15. Accordingly, the strip isable to hang in the loop 36 as it passes from the casting rolls 22 tothe guide table 13 and into the pinch roll stand 14. The pinch rolls 50thus provides a tension barrier between the freely hanging loop andtension on the strip downstream of the processing line. The pinch rolls50 also stabilize the position of the strip on the feed table 13,feeding the strip into hot rolling mill 15.

From the pinch roll stand 14, the thin cast strip 12 is delivered to thehot rolling mill 15 comprised of upper work roll 16A and lower roll 16B.As shown in FIG. 4, a preferred embodiment of the present inventioncomprises spraying a mixture of water and oil on the downstream surfacesof back-up rolls 16. An oil reservoir 100 is provided with a heater 101to maintain the oil at approximately 50° C., but heating is notnecessary. The heated oil is transferred through oil transfer lines 103by fixed displacement pumps 102 to static mixers 104 where the heatedoil is mixed with water.

Water is supplied from a source 110 to water strip chilling headers 111and to mill rolls supply lines 112. A first portion of the water issupplied to spray headers 18 to supply cooling water to cool the hotstrip 12 after exiting the hot rolling mill 15. Typically, the waterpressure is reduced through pressure regulator 113 to about 40 psi.Between about 10 and 30 gpm of water is supplied to each static mixer104 where the water is mixed with about 4 gph of heated oil.

The mixed oil and water is then applied to the downstream surfaces (thedirection of travel of the thin cast steel strip 12 is shown by arrow120) of back-up rolls 16 through oil-water nozzles 71. Alternately, theoil-water mixture may be applied to cast strip 12 in the roll bite area,may be applied to the upstream surfaces of the back-up rolls 16 or tothe work rolls 16A, 16B.

Preferably, the temperature of the thin cast steel strip 12 in the hotrolling mill 15 is less than 1100° C., and more preferably less than1050° C., and most preferably less than 900° C. Also, preferably, thetemperature of the thin cast steel strip in the hot rolling mill 15 isabove 400° C.

The static mixers 104 are standard conventionally available devices.Other forms of mixers may be used provided they are capable of goodmixing of the oil and water.

In one embodiment, the oil-water mixture is delivered at between 5 and30 gpm at 40 psi to the back-up rolls 16. Typically the oil-watermixture is delivered to the back up rolls in this embodiment at about 10to 20 gpm, with 15 gpm a reasonable setting. The oil-water mixture maycomprise less than 5% oil, and in one embodiment comprises 4 parts oiland between 600 parts to 1800 parts water by volume. The oil may be lessthan 2% or 1% of the mixture. The oil is provided to be mixed with thewater generally at less than 15 gph.

FIG. 5 shows the Average Surface Roughness (Ra) in microns for thin caststrip steel strip 12 produced using the present invention. As can beseen in FIG. 5, the Average Surface Roughness is noticeably lower, about0.66 to about 1.5 microns with the addition of an oil-water mixture asdescribed above.

In one embodiment, the present invention comprises producing thin caststeel strip using the oil-water application described above to producethin cast steel strip at a rate above 80 meters per minute.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from its scope.Therefore, it is intended that the invention not be limited to theparticular embodiments disclosed, but that the invention will includeall embodiments falling within the scope of the appended claims.

1. A thin cast strip having at least one microstructure selected fromthe group consisting of polygonal ferrite, acicular ferrite,Widmanstatten, bainite and martinsite, a surface roughness of less than1.5 microns Ra and a scale thickness of less than about 10 microns madeby the steps comprising: a. assembling a twin roll caster havinglaterally positioned caster rolls forming a nip between them and a hotrolling mill having work rolls and back-up rolls adjacent the twin rollcaster, b. forming a thin cast strip from the nip between the castingrolls of the twin roll caster, c. applying a mixture of water and oil onthe work rolls of the hot rolling mill, d. passing the thin cast stripat a temperature of less than 1100° C. through the hot rolling millwhile the mixture of oil and water is applied to the work rolls, and e.shrouding the thin cast strip from the casting rolls through the hotrolling mill in an atmosphere of less than 5% oxygen forming a thin caststrip having: at least one microstructure selected from the groupconsisting of polygonal ferrite, acicular ferrite, Widmanstatten,bainite and martinsite, a surface roughness of less than 1.5 microns Ra,and a scale thickness of less than 10 microns.
 2. A thin cast strip asdescribed in claim 1 wherein the thin cast strip is passed through thehot rolling mill at a temperature less than 1050° C. while the mixtureof oil and water is applied to the work rolls.
 3. A thin cast strip asdescribed in claim 1 wherein the surface roughness is less than 1.0microns Ra.
 4. A thin cast strip as described in claim 1 wherein thesurface roughness is less than 0.7 microns Ra.
 5. A thin cast strip asdescribed in claim 1 wherein the surface roughness is less than 0.5microns Ra.
 6. A thin cast strip as described in claim 1 wherein thescale thickness is less than 7 microns.
 7. A thin cast strip asdescribed in claim 1 wherein the scale thickness is less than 4 microns.8. A thin cast strip as described in claim 1 wherein the mixture of oiland water is applied by spying the work rolls.
 9. A thin cast strip asdescribed in claim 1 wherein the mixture of oil and water is applied tothe work rolls by applying the mixture of oil and water to the back-uprolls.
 10. A thin cast strip as described in claim 1 wherein the mixtureof oil and water is less than 5% oil and the low surface roughness isless than 1.5 microns Ra.
 11. A thin cast steel strip with reducedsurface roughness below 1.5 microns Ra, produced by the stepscomprising: a. assembling a strip caster having a pair of casting rollshaving a nip there between; b. assembling a metal delivery systemcapable of forming a casting pool between the cast rolls above the nipwith side dams adjacent the ends of the nip to confine said castingpool; c. assembling adjacent the strip caster a hot rolling mill havingwork rolls with work surfaces forming a gap between them through whichhot strip is rolled; d. assembling spray nozzles positioned adjacent thework rolls capable of providing a mixture of water and oil to the workrolls; e. introducing molten steel between the pair of casting rolls toform a casting pool supported on casting surfaces of the casting rollsconfined by said first side dams; f. counter-rotating the casting rollsto form solidified metal shells on the surfaces of the casting rolls andcast steel strip through the nip between the casting rolls from saidsolidified shells; g. spraying the mixture of oil and water as the stripenters the hot rolling mill; and h. rolling the cast strip between thework rolls of the hot rolling mill to produce a cast strip having asurface roughness less than 1 microns Ra.
 12. The thin cast steel stripwith reduced surface roughness as claimed in claim 11 where the rate ofproduction of the cast strip in above 80 meters per minute.
 13. The thincast steel strip with reduced surface roughness as claimed in claim 11where the rolling temperature is below 900° C.
 14. The thin cast steelstrip with reduced surface roughness as claimed in claim 11 where therate of spray by the nozzles is between 10 and 30 gallons per minute.15. The thin cast steel strip with reduced surface roughness as claimedin claim 11 where the surface roughness below 0.7 microns Ra.
 16. Thethin cast steel strip with reduced surface roughness as claimed in claim11 where the surface roughness below 0.4 microns Ra.
 17. The thin caststeel strip with reduced surface roughness as claimed in claim 11 wherethe rate of production of the cast strip in above 80 meters per minute.18. A thin cast steel strip with reduced surface roughness below 1.5microns Ra, produced by the steps comprising: a. assembling a stripcaster having a pair of casting rolls having a nip there between; b.assembling a metal delivery system capable of forming a casting poolbetween the cast rolls above the nip with side dams adjacent the ends ofthe nip to confine said casting pool; c. assembling adjacent the stripcaster a hot rolling mill having back-up rolls and work rolls with worksurfaces forming a gap between the work rolls through which hot strip isrolled; d. assembling spray nozzles positioned upstream of the workrolls capable of spraying a mixture of water and oil the back-up rolls;e. introducing molten steel between the pair of casting rolls to form acasting pool supported on casting surfaces of the casting rolls confinedby said first side dams; f. counter-rotating the casting rolls to formsolidified metal shells on the surfaces of the casting rolls and caststeel strip through the nip between the casting rolls from thesolidified shells; g spraying the mixture of oil and water as the caststrip enters the hot rolling mill; and h. rolling the cast strip betweenthe work rolls of the hot rolling mill to produce a cast strip having asurface roughness less than 1.5 microns Ra.
 19. The thin cast steelstrip with reduced surface roughness below 1.5 microns Ra as claimed inclaim 18 where the rate of production of the cast strip in above 80meters per minute.
 20. The thin cast steel strip with reduced surfaceroughness below 1.5 microns Ra as claimed in claim 18 where the rollingtemperature is below 10° C.
 21. The thin cast steel strip with reducedsurface roughness as claimed in claim 18 where the rolling temperatureis below 1050° C.
 22. The thin cast steel strip with reduced surfaceroughness as claimed in claim 18 where the rolling temperature is below900° C.
 23. The thin cast steel strip with reduced surface roughness asclaimed in claim 18 where the rate of spray by the nozzles is between 10and 30 gallons per minute.
 24. The thin cast steel strip with reducedsurface roughness as claimed in claim 18 where the surface roughnessbelow 0.7 microns Ra.
 25. The thin cast steel strip with reduced surfaceroughness as claimed in claim 24 where the rate of production of thecast strip in above 80 meters per minute.
 26. The thin cast steel stripwith reduced surface roughness as claimed in claim 24 where the rollingtemperature is below 1100° C.
 27. The thin cast steel strip with reducedsurface roughness as claimed in claim 24 where the rolling temperatureis below 1050° C.
 28. The thin cast steel strip with reduced surfaceroughness as claimed in claim 24 where the rolling temperature is below900° C.
 29. The thin cast steel strip with reduced surface roughness asclaimed in claim 24 where the rate of spray by the nozzles is between 10and 30 gallons per minute.
 30. The thin cast steel strip with reducedsurface roughness as claimed in claim 18 where the mixture of oil andwater is less than 5% oil.
 31. The thin cast steel strip with reducedsurface roughness below 1 microns Ra as claimed in claim 18 where thethin cast steel strip has a surface scale thickness of less than about 7microns.